Methods for an expandable covered stent with wide range of wrinkle-free deployed diameters

ABSTRACT

An improved stent-graft device is provided that delivers a smooth flow surface over a range of operative expanded diameters by applying a unique cover material to the stent through a technique that allows the cover to become wrinkle-free prior to reaching fully deployed diameter. The unique cover material then allows the device to continue to expand to a fully deployed diameter while maintaining a smooth and coherent flow surface throughout this additional expansion. Employed with a self-expanding device, when the device is unconstrained from a compacted diameter it will self-expand up to a fully deployed diameter with the graft being substantially wrinkle-free over diameters ranging from about 30-50% to 100% of the fully deployed diameter.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/408,474 filed on Apr. 21, 2006, which is incorporated by referenceherein in its entirety.

FIELD

The present invention relates to covered stents for use in variousmedical procedures.

BACKGROUND

The following terms used herein are defined as follows:

The term “stent” means a frame structure containing openings through itswall, typically cylindrical in shape, intended for implantation into thebody. A stent may be self-expanding and/or expanded using appliedforces.

As used herein, the terms “covered stent” and “stent-graft” are usedinterchangeably to mean a stent with a cover on at least a portion ofits length. The cover can be on the outer surface, the inner surface, onboth surfaces of the stent, or the stent may be embedded within thecover itself. The cover may be porous or non-porous and permeable ornon-permeable. Active or inactive agents or fillers can be attached toor incorporated into the cover.

Referring to FIG. 4 a, as used in this application, the term “wrinkle”65 a, 65 b means a fold in a stent cover 62 that has a larger peak tovalley height 64 than a thickness 66 of an adjacent stent strut 68. Inthe illustrated instance where the cover is mounted within the stent, awrinkle 65 a in a cover 62 on the outer surface of a covered stent 60may be identified where the cover extends beyond the inner surface ofthe stent struts 68. A wrinkle 65 b can also extend radially inward.

Referring to FIG. 4 b, a wrinkle 65 a in a cover on the inner surface ofa covered stent 60 can extend radially outward. Such anoutward-extending wrinkle may be identified where the cover 62 extendsbeyond the outer surface of the stent struts 68 as shown in FIG. 4 b. Awrinkle 65 b can also extend radially inward as shown in FIG. 4 b

Wrinkles can be observed with unaided vision or they can be observed andmeasured under magnification, such as optical microscopy. “Wrinkle-free”means a stent covering that is substantially free of “wrinkles.”

As used herein, the term “expand” has two distinct meanings. When usedin the context of describing stents, it refers to the increase indiameter of those devices. When used in the context of ePTFE material,it refers to the stretching (i.e., expansion) process used to renderPTFE material stronger and porous.

As used herein, the term “self-expanding” means the attribute of adevice that describes that it expands outwardly, such as in a generalradial direction, upon removal of a constraining means, therebyincreasing in diameter without the aid of an external force. That is,self-expanding devices inherently increase in diameter once aconstraining mechanism is removed. Constraining means include, but arenot limited to, tubes from which the stent or covered stent device isremoved, such as by pushing. Alternatively, a constraining tube orsheath may be disrupted to free the device or the constraining means canbe unraveled should it be constructed of a fiber or fibers. Externalforces, as provided by balloon catheters for example, may be used priorto expansion to help initiate an expansion process, during expansion tofacilitate expansion, and/or after stent or covered stent deployment tofurther expand or otherwise help fully deploy and seat the device.

As used herein, the term “fully deployed” refers to the state of aself-expanding stent after which the constraining means has been removedand the stent, at about 37° C. over the course of 30 seconds, hasexpanded under its own means without any restriction. A portion orportions of a self-expanding stent may be fully deployed and theremainder of the stent may be not fully deployed.

The phrase, “operating diametric range” refers to the diametric sizerange over which the stent or stent-graft will be used and typicallyrefers to the inner diameter of the device. Devices are frequentlyimplanted in vessel diameters smaller than that corresponding to thedevice fully deployed state. This operating range may be the labeledsize(s) that appear in the product literature or on the product packageor it can encompass a wider range, depending on the use of the device.

As used herein, the term “porous” describes a material that containssmall or microscopic openings, or pores. Without limitation, “porous” isinclusive of materials that possess pores that are observable undermicroscopic examination. “Non-porous” refers to materials that aresubstantially free of pores. The term “permeable” describes a materialthrough which fluids (liquid and/or gas) can pass. “Impermeable”describes materials that block the passage of fluids. It should beappreciated that a material may be non-porous yet still be permeable tocertain substances.

Stents and covered stents have a long history in the treatment oftrauma-related injuries and disease, especially in the treatment ofvascular disease. Stents can provide a dimensionally stable conduit forblood flow. Stents prevent vessel recoil subsequent to balloondilatation thereby maintaining maximal blood flow. Covered stents canprovide the additional benefits of preventing blood leakage through thewall of the device and inhibiting, if not preventing, tissue growththrough the stent into the lumen of the device. Such growth through theinterstices of the stent may obviate the intended benefits of thestenting procedure.

In the treatment of carotid arteries and the neurovasculature, coveringstrap plaque particles and other potential emboli against the vessel wallthereby preventing them from entering the blood stream and possiblycausing a stroke. Coverings on stents are also highly desirable for thetreatment of aneurismal vascular disease. The covers may further act asuseful substrates for adding fillers or other bioactive agents (such asanticoagulant drugs, antibiotics, growth inhibiting agents, and thelike) to enhance device performance.

The stent covers may extend along a portion or portions or along theentire length of the stent. Generally, stent covers should bebiocompatible and robust. They can be subjected to cyclic stresses abouta non-zero mean pressure. Consequently, it is desirable for them to befatigue and creep resistant in order to resist the long-term effects ofblood pressure. It is also desirable that stent covers be wear-resistantand abrasion-resistant. These attributes are balanced with a desire toprovide as thin a cover as possible in order to achieve as small adelivery profile as possible. Covers compromise the flow cross-sectionof the devices, thereby narrowing the blood flow area of the device,which increases the resistance to flow. While increased flow area isdesirable, durability can be critical to the long-term performance ofcovered stents. Design choice, therefore, may favor the stronger, hencethicker, covering. Thick covers, however, are more resistant todistension than otherwise identical thinner covers.

Some balloon-expandable stent covers are wrinkle-free over the operatingrange of the stents because the extreme pressures of the balloons candistend the thick, strong covers that are placed onto the stent at aless than a fully deployed stent diameter. Even the thinnest covers inthe prior art such as those made of ePTFE (e.g., those taught in U.S.Pat. No. 6,923,827 to Campbell et al., and U.S. Pat. No. 5,800,522 toCampbell et al.), however, may be too unyielding to be distended by theradial forces exerted by even the most robust self-expanding stents.

Non-elastic and non-deformable self-expanding stent covers are,therefore, generally attached in a wrinkle-free state to the stent whenthe stent is fully deployed. When such covered stents are at any outerdiameter smaller than the fully deployed outer diameter, the cover isnecessarily wrinkled. These wrinkles, unfortunately, can serve as sitesfor flow disruption, clot initiation, infection, and other problems. Thepresence of wrinkles may be especially deleterious at the inlet tocovered stents. The gap between the wrinkled leading edge of the coverand the host vessel wall can be a site for thrombus accumulation andproliferation. The adverse consequences of wrinkles are particularlysignificant in small diameter vessels which are prone to fail due tothrombosis, and even more significant in the small vessels that provideblood to the brain.

The use of thin, strong materials is known for implantable devices(e.g., those taught in U.S. Pat. No. 5,735,892 to Myers et al.).Extremely thin films of expanded PTFE (ePTFE) have been taught to coverboth self-expanding and balloon expandable stents. Typically these filmsare oriented during the construction of the devices to impart strengthin the circumferential direction of the device. Consequently, theexpanding forces of the self-expanding stents may be far too low todistend these materials. In fact, such devices are generally designed towithstand high pressures. These coverings, like those of other coveringsin the art, are wrinkle-free only when the devices are fully deployed.

Thin, extruded but not expanded fluoropolymer tubes have been used tocover self-expanding and balloon-expandable stents (e.g., U.S. PatentApplication 2003/0082324 A1 to Sogard). These seamless extruded tubecovers are applied to self-expanding stents in the fully deployed stateof the stents. The stent coverings, therefore, possess wrinkles uponcrushing the device to a diameter smaller than the fully deployeddiameter.

Expanded PTFE material has been used to cover stents that areself-expanding up to a given diameter, then use the assistance of aballoon catheter or other expansion force to achieve the desiredclinical implantation diameter (e.g., U.S. Pat. No. 6,336,937 to Voneshet al). Such covers are wrinkled in the range of diameters up to thediameter at which the stent expands on its own. Beyond that diameter,the covers may be relatively wrinkle-free, however, the stent may nolonger be freely self-expanding.

Another type of covered stent previously disclosed (e.g., U.S. PatentApplication 2002/0178570 A1 to Sogard) is constructed with two polymericliners laminated together yet not adhered to the stent. In the absenceof bonding a liner to the stent, both an inner and outer liner arenecessary and they need to be bonded together at the stent openings inorder to construct a coherent stent-graft. This construction provides arelatively smooth liner on one side of the stent. The outer linerfollows the geometry of the stent strut and is bonded to the innerliner. As such, according to the definition of a “wrinkle” as providedherein, the outer liner is wrinkled. Expanded PTFE liners ofself-expanding covered stents made with shape memory alloys were taughtto be laminated together at elevated temperatures, as high as 250° C.(and below 327° C.), while not exceeding a stent temperature which mightreset the shape memory state of the alloy. In the absence of bonding theliners to the stent struts, gaps are formed between the liners. Suchgaps may become filled with biological materials that compromise theblood flow area and, therefore, may restrict blood flow.

Without the addition of other materials, expanded PTFE materials must beheated well above 200° C. in order the heat bond them together. Giventhat these stent-graft devices are intended to self-expand at bodytemperature, the temperature at which the alloy may reset is necessarilyclose to body temperature. This thermal requirement obviates thepossibility of heat bonding the liner to the stent at around a 250° C.temperature. Furthermore, the size of the covered stent that can beconstructed in this manner is limited by the physics of heat conduction.That is, a 250° C. heat source must be at a suitable distance from thestent during the lamination process. The liners are laminated with thestent at a diameter less than deployed diameter, hence the size of theopenings of the stent are smaller than if the liners were laminated at alarger stent diameter. Consequently, small diameter covered stentscannot be made in accordance with these teachings, nor can the liners bebonded to the stent.

U.S. Pat. No. 6,156,064 to Chouinard teaches use of dip coating to applypolymers to self-expanding stents. Stents and stent-grafts are dippedinto polymer-solvent solutions to form a film on the stent followed byspray coating and applying a polymeric film to the tube. Stent-graftscomprising at least three layers (i.e., stent, graft, and membrane) aretaught to be constructed in this manner.

Stents have also been covered with a continuous layer of elasticmaterial. As taught in U.S. Pat. No. 5,534,287 to Lukic, a covering maybe applied to a stent by radially contracting the stent, then placing itinside a tube with a coating on its inner surface. The stent is allowedto expand, thereby bringing it in contact with the coating on the tube.The surface of contact between the stent and the tube is then vulcanizedor similarly bonded. No teaching is provided concerning the diameter ofthe tube relative to the fully deployed stent diameter. The patentspecifically teaches in one embodiment the application of the coating ona stent in the expanded condition. The inventor does not teach how toeliminate or even reduce wrinkles in the stent cover. In fact, thepatent teaches how to increase the thickness of the coating, a processthat would only increase the occurrence of wrinkling. The patent teachesaway from the use of a non-elastic material to cover the stent, andspecifically teaches away from the use of a “Teflon®” (i.e., PTFE) tube.

U.S. Patent Application 2004/0024448 A1 to Chang et al teaches coveredstents with elastomeric materials including PAVE-TFE. Self-expandingstent-grafts made with this material, like those made of other materialsin the art, are not wrinkle-free over the operating range of thedevices. These coverings of self-expanding stents are typically appliedto the stent in the fully-deployed state. Consequently, wrinkles areformed when the stent-graft is crushed to any significant degree.

SUMMARY

The present invention is an improved expandable implantable stent-graftdevice that provides a smooth flow surface over a range of operativeexpanded diameters. This is accomplished by applying a unique covermaterial to the stent through a unique technique that allows the coverto become wrinkle-free prior to reaching fully deployed diameter. Theunique cover material then allows the device to continue to expand to afully deployed diameter while maintaining a smooth and coherent flowsurface throughout this additional expansion.

In one embodiment the present invention comprises a diametricallyself-expanding stent-graft device having a graft covering attached to atleast a portion of the stent. The device is adapted to be constrainedinto a compacted diameter for insertion into a body conduit, which willproduce wrinkles along its graft surface. However, when the device isunconstrained from the compacted diameter it will self-expand up to afully deployed diameter with the graft being substantially wrinkle-freeover diameters ranging from 50% to 100% of the fully deployed diameter.

Further improvements in the present invention may include providing afluoropolymer graft component, such as an ePTFE, in the form of either acoherent continuous tube or a film tube. The graft and stent may becombined together through a variety of means, including using heatbonding or adhesive, such as FEP or PMVE-TFE.

By modifying the materials and/or the construction techniques, the rangeof wrinkle-free expansions can be increased to about 30%-100% or evenwider ranges.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a three-quarter isometric view of one embodiment of acovered stent of the present invention in the constrained state, havingthe cover mounted on the outside of the stent;

FIG. 1 b is a three-quarter isometric view of the embodiment of acovered stent of the present invention of FIG. 1 a in the fully deployedstate;

FIG. 2 a is a transverse cross-section view of the embodiment of acovered stent of the present invention deployed to 30% of the fullydeployed outer diameter of the device;

FIG. 2 b is a transverse cross-section view of the embodiment of acovered stent of the present invention deployed to 50% of the fullydeployed outer diameter of the device with the smooth gradual transitionof the adhesive-stent cover interface shown in detail in an enlargedsectional view;

FIG. 2 c is a transverse cross-section view of the embodiment of acovered stent of the present invention taken along line 2 c-2 c of FIG.1 b, deployed to 100% of the fully deployed outer diameter of the devicewith the smooth gradual transition of the adhesive-stent cover interfaceshown in detail in an enlarged sectional view;

FIG. 3 a is a photomicrograph showing the inside of a covered stent ofthe present invention that is constrained in a partially deployed stateof about 50% of the fully deployed outer diameter of the device;

FIG. 3 b is a photomicrograph showing the inside of a covered stent ofthe present invention that is constrained in a partially deployed stateof about 60% of the fully deployed outer diameter of the device;

FIG. 3 c is a photomicrograph showing the inside of a covered stent ofthe present invention that is constrained in a partially deployed stateof about 70% of the fully deployed outer diameter of the device;

FIG. 3 d is a photomicrograph showing the inside of a covered stent ofthe present invention that is constrained in a partially deployed stateof about 80% of the fully deployed outer diameter of the device;

FIG. 3 e is a photomicrograph showing the inside of a covered stent ofthe present invention that is constrained in a partially deployed stateof about 90% of the fully deployed outer diameter of the device;

FIG. 3 f is a photomicrograph showing the inside of a covered stent ofthe present invention that is fully deployed;

FIG. 3 g is a photomicrograph showing the inside of a covered stent ofthe prior art that is constrained in a partially deployed state of about50% of the fully deployed diameter;

FIG. 4 a is a transverse cross-section view of exemplary wrinkles in acover on the outer surface of the stent; and

FIG. 4 b is a transverse cross-section view of exemplary wrinkles in acover on the inner surface of the stent.

DETAILED DESCRIPTION

The present invention addresses the problem of wrinkles in the covers instent-grafts. The covers of self-expanding stent-grafts heretoforeexhibited wrinkles when deployed to diameters smaller than the diameterat which the cover was applied to the stent, which is typically thefully deployed diameter. Inasmuch as body conduits are rarely the exactdiameter of the stent-graft, rarely uniformly circular in cross-section,and rarely non-tapered, sections or entire lengths of self-expandingstent-grafts frequently are not fully deployed and hence presentwrinkled surfaces to flowing blood or other body fluids. Furthermore,covered stents are often intentionally implanted at less than theirfully deployed diameters in order to utilize their inherent radialexpansion force to better anchor the devices against the host tissue,thereby preventing device migration in response to blood flow. Suchpractices come at the expense of having to tolerate devices with atleast partially wrinkled covers. The present invention involves the useof a unique stent cover material, one that combines two seeminglymutually exclusive properties—being both strong enough to withstand theforces exerted by constant, cyclic blood pressure and also distensibleenough to expand in response to the expansion forces exerted by aself-expanding stent.

In addition, a unique manufacturing method had to be devised in order toutilize this material to construct a self-expanding stent-graft. Thetemperature-constrained shape-memory properties of self-expanding stentsintroduce significant processing challenges. Ultimately, a process wasdeveloped which entailed not only applying the cover to the stent in acold environment, but also entailed bonding the cover to the stent atthese cold temperatures.

Referring to FIGS. 1 a and 1 b, the present invention is directed toimplantable device 60 having a self-expanding stent component 63 witheither an inner or outer cover 62 (or both), that is wrinkle-free overan operating diametric range of the device. The cover 62 has wrinkles 65in the constrained state as shown in FIG. 1 a. The wrinkles disappearonce the device self-expands to the diameter at which the cover wasapplied to the stent. The cover 62 remains wrinkle-free as the device 60self-expands even further as shown in FIG. 1 b. The invention addressesthe clinical problems associated with wrinkles in self-expanding stentcovers while providing the minimum amount of covering material. Wrinklesare known to disrupt blood flow and become sites for clot depositionwhich can ultimately lead to graft thrombosis and embolus shedding.These sequelae may create serious clinical consequences, especially inorgans such as the brain. The incorporation of a single, very thin coverenables a stent-graft device with a profile dictated primarily by thestent strut dimensions, not by the mass or volume of the cover. Thepresent invention, therefore, provides a heretofore unavailablecombination of deployment diameter for a given size stent-graft and awrinkle-free cover surface over a wide range of deployed diameters.

For use in the present invention, nitinol (nickel-titanium shape memoryalloy) and stainless steel are preferred stent materials. Nitinol ispreferred for its shape memory properties. The memory characteristicscan be tailored for the requirements of the stenting application duringthe fabrication of the alloy. Furthermore, nitinol used to make thestent can be in the form of wire that can be braided or welded, forexample, or it can be tubing stock from which a stent is cut. Whilenitinol offers a wide variety of stent design options, it should beappreciated that stainless steel and other materials may also be formedinto many different shapes and constructs.

Stent covers of the present invention are preferably durable andbiocompatible. They may be seamless or contain one or more seams. Thestent covering of the present invention has a low Young's modulus, whichenables it to be distended with the minimal force that is exerted by aself-expanding stent. Furthermore, the covering is provided with aminimal (or non-existent) elastic recoil force so that after stentexpansion the covering does not cause the stent-graft to decrease indiameter over time. The cover is also preferably thin. Thinness has themultiple benefits of reducing the introduction size of the device,maximizing the blood flow cross-section, providing less resistance toradial expansion, and introducing less elastic recoil.

In a preferred embodiment, a nitinol stent is chilled and crushed to adiameter less than the fully deployed outer diameter. The chilling isdesirable to help maintain the stent in the crushed state. The coveringis then applied without creating wrinkles. The constrained diameter isselected according to the intended operating parameters of the device,such as about 90% of the fully deployed outer diameter or less, about80% of the fully deployed outer diameter or less, about 70% of the fullydeployed outer diameter or less, about 60% of the fully deployed outerdiameter or less, and for most applications most preferably about 50% ofthe fully deployed outer diameter or less. While maintaining the devicein the chilled state, the stent-graft is allowed to dry and then furthercrimped with a chilled crimping tool and transferred into a deliverycatheter.

The stent cover may consist of fluorinated ethylene propylene (FEP)coating the nodes and fibrils of ePTFE film. Most preferably, a cover ofePTFE, is used to practice the invention. Whereas ePTFE is known for itshigh tensile strength, that strength is imparted only in the directionof expansion. If the ePTFE material is not expanded in the orthogonaldirection (i.e., the transverse direction in the case of films) duringthe processing of the material, the ePTFE material is extremelydistensible in that direction. Such materials have both very low tensilestrength and very low Young's modulus in the transverse direction. Thelow Young's modulus property enables the material to distend under lowforces. Films used to construct articles of the present invention can beeasily elongated in the transverse direction by hand, therebydemonstrating their low Young's modulus values. In the most preferredembodiments, therefore, the ePTFE materials are in the form of verythin, highly porous films that are highly distensible in the transversedirection. The combination of high porosity and thinness result in acover material that occupies minimal volume of the device. Expanded PTFEstent covers may offer additional advantages by virtue of the ability toprovide and control their porosity. Various agents or fillers can beadded to the surface or within the pores of the material. Such agentsand fillers may include but are not limited to therapeutic drugs,antithrombotic agents, and radio opaque markers. If desired, portions ofor the entire ePTFE cover may optionally be rendered non-porous ornon-permeable by densifying, filling the pores, or through any othersuitable means. Preferably, to provide added stability to the material,the ePTFE material is raised above its crystalline melt point, that is,the ePTFE material is “sintered.”

It is believed that thin ePTFE films possessing a thickness of less thanabout 0.25 mm are preferred for practicing the present invention. It isbelieved that even more preferred are films possessing a thickness lessthan about 0.1 mm. Preferred thin ePTFE films possess densities in therange of about 0.2 to about 0.6 g/cc. It is believed that more preferredthin ePTFE films have densities in the range of about 0.3 to about 0.5g/cc. It is believed that preferred thin ePTFE films possess matrixtensile strengths in the range of about 70 to about 550 MPa and about 15to about 50 MPa, in the longitudinal and transverse directions,respectively. It is believed that more preferred thin ePTFE filmspossess matrix tensile strengths in the range of about 150 to about 400MPa and about 20 to about 40 MPa, in the longitudinal and transversedirections, respectively. The preferred film for use in practicing thepresent invention is a thin ePTFE film possessing a thickness of about0.02 mm, a density of about 0.4 g/cc, longitudinal matrix strength ofabout 260 MPA, and a transverse matrix tensile strength of about 30 MPa.

It is believed that preferred thin ePTFE films possess Young's modulusin the range of about 100 to about 500 MPa and about 0.5 to about 20MPa, in the longitudinal and transverse directions, respectively. It isbelieved that more preferred thin ePTFE films possess Young's modulus inthe range of about 200 to about 400 MPa and about 1 to about 10 MPa, inthe longitudinal and transverse directions, respectively. The mostpreferred Young's modulus values of the film in the longitudinal andtransverse directions are about 300 MPa and about 2 MPa, respectively.This film is exceedingly distensible in the transverse direction.

The choice of film properties is largely dependent on the force theself-expanding stent exerts on the material during expansion. Forexample, stronger films may be used with stents that exert higher radialforces during self-expansion.

To take advantage of the low Young's modulus of the film, the coveredstent may be constructed with the low Young's modulus direction of thefilm oriented in the circumferential direction of the stent. The highstrength direction of the film is therefore oriented in the axialdirection of the stent. Preferably, the film is applied to the stent inthe shape of a tube. A film tube is constructed by rolling multiplelayers of the film around the circumference of a mandrel that is coveredwith a release material (such as Kapton film, part number T-188-1/1,Fralock Corporation, Canoga Park, Calif.). Preferably, three or fewerePTFE film layers are applied, more preferably a single layer is appliedwherein the overlap seam is narrow and comprises only two layers of thefilm.

The film tube can be attached to the stent by suturing, gluing, and thelike. Gluing is preferred, utilizing an adhesive or combination ofadhesives by means such as spraying or dipping. It is preferred to dipcoat a fully deployed stent with an adhesive, ensuring that the adhesivedoes not span the openings in the stent. Thermal or ambient curedadhesives can be used. When bonding the film tube to a shape memorymetal stent using a thermally-activated adhesive, the adhesive should becurable at a temperature below the critical transition temperatures ofthe metal. Adhesives such asperfluoroethylvinylether-tetrafluoroethylene (PEVE-TFE) orperfluoropropylvinylether-tetrafluoroethylene (PPVE-TFE) are preferred.Terpolymers containing at least two of the following monomers are alsopreferred: PEVE, PPVE, perfluoromethylvinylether (PMVE), and TFE. Mostpreferably, the adhesive isperfluoromethylvinylether-tetrafluoroethylene (PMVE-TFE) material whenbonding the cover to a nitinol stent.

FIG. 2 a depicts a cross-section of the covered stent of the presentinvention that was constructed at 50% of the fully deployed outerdiameter, crimped and transferred inside a delivery catheter, and thendeployed to 30% of the fully deployed outer diameter of the device. Thestent cover 62 can be attached to the outer surface of the stent bybonding it to stent struts 68 as shown in FIG. 2 a, thereby providing anouter stent cover 51 to the stent 63. The cover 62 can alternatively bebonded to the inner surface of the stent as shown in FIG. 4 b, providingan inner stent cover 41.

The most preferred way to attach the film tube to the outer surface ofthe stent involves placing the film tube inside a rigid (e.g., glass)tube that has an inner diameter smaller than the fully deployed outdiameter of the stent, then inserting the crimped stent inside the filmtube and bonding the stent and film tube together.

The film tube covering is first inserted inside the constraining tubewithout creating wrinkles. The ends of the film tube may be everted overthe ends of the constraining tube. Preferably the ends are everted tothe extent that modest tension is applied to the film tube, enough tohold the film tube taut and thereby keep the film tube free of wrinkles.As has been noted, the inner diameter of the constraining tube, andhence the constraining diameter, should be less than the fully deployeddiameter of the device, such as 90% of the fully deployed outer diameteror less, about 80% of the fully deployed outer diameter or less, about70% of the fully deployed outer diameter or less, about 60% of the fullydeployed outer diameter or less, or about 50% of the fully deployedouter diameter or less.

A nitinol stent is prepared by dip coating a thin layer of adhesive toits struts and allowing the adhesive to dry. The preferred adhesive isPMVE-TFE, such as that taught in Example 5 of US Patent Application2004/0024448 to Chang et al. Contrary to practices in the prior art thatteach bonding covers to stents at ambient or even highly elevatedtemperatures, the cover is applied to the stent at lower than ambienttemperatures. Preferably, the stent is chilled and crimped in a coldchamber (e.g., the freezer compartment of a refrigerator). The lowtemperature process is desired in order to cool the stent in order todimensionally stabilize it at a diameter less than the film tubediameter while the cover is attached. The crimped stent is next insertedinside the film tube, which is inside a rigid tube. The assembly ispermitted to warm to ambient temperature. The stent expands, hence comesin intimate contact with the film tube, as it warms. The assembly issubmerged in a solvent that activates the PMVE-TFE adhesive and thenwarmed above ambient temperature to evaporate the solvent, thus allowingthe adhesive to solidify.

The device inside the rigid tube is then again chilled in a freezer to atemperature at which at the device does not self-expand if unconstrainedand then the stent-graft is removed from the tube. At this point, thestent-graft is further crimped using the chilled crimping machine, andtransferred inside of a delivery catheter. Instead of crimping at thisstage, alternatively the porous ePTFE cover of the stent-graft devicemay be rendered non-permeable. One method to do so can be achieved bydipping the device into a chilled dilute solution of elastomericmaterial, such as PMVE-TFE, PEVE-TFE, PPVE-TFE, or silicone. A dilutesolution is preferred inasmuch as the solution becomes significantlymore viscous when chilled to the same temperature as the device. Oncethe solution dries, the stent-graft can be crimped further, aspreviously described, and transferred inside of a delivery catheter.

Therapeutic agents, fillers, or the like can be added to the stentcover, the adhesive used to bond the stent cover to the stent or theelastomer material used to render the cover non-permeable or anycombination thereof.

Stent-grafts made in this manner exhibit wrinkle-free coverings over thedevice diameter range extending from the diameter at which the coveringwas applied up to and including the fully deployed diameter. FIG. 2 billustrates the wrinkle-free stent cover 62 (in this case, on the outersurface of the stent) at the diameter at which it was bonded to thestent struts 68, thereby forming the covered stent device 60. The thincover 62 stretches and remains wrinkle free up to and including thefully deployed diameter as shown in FIG. 2 c. FIG. 2 c depicts across-section of the covered stent of FIG. 1 b. In order to achieve thisdevice performance, the covering should be applied to the stent at adiameter smaller than the fully deployed diameter. This diameter shouldbe no larger than the smallest intended diameter of the implanteddevice. Crushing the device below the diameter at which the cover wasapplied induces wrinkles in the stent cover. For example, crushing adevice of the present invention to such a degree that it is small enoughto be transferred to inside a delivery catheter will induce wrinkles inthe stent cover. The wrinkles are no longer present once the deployedstent-graft reaches the diameter at which the cover was applied.Attaching the covering at an intermediate stent size means less crushingis necessary to decrease the stent-graft diameter for insertion into thedelivery catheter. The likelihood of perforating the cover during thecrushing process is reduced when less crushing is needed.

A stent-graft with an inner cover can be fabricated with a film tube andan adhesive-coated stent as previously described. The stent can bechilled then crushed and constrained inside a constraining tube. Thefilm tube can then be mounted onto a balloon, introduced inside thestent, pressed against the stent via inflating the underlying balloon,then bonded to the stent by immersing the assembly into the appropriatesolvent for the adhesive, and then allowed to dry. The balloon is thendeflated and the stent-graft plus the constraining tube are againchilled to enable removal of the constraining tube prior to furtherradial crushing of the stent-graft and loading the device into thedelivery system.

The present invention also minimizes flow disturbances caused by bluntstent strut profiles. As seen in FIG. 2 b and FIG. 2 c the adhesivematerial 22 bonded to stent strut 68 forms a smooth gradual transitionwhere it attaches to stent cover 62. In the absence of this transition,the stent strut 68 may present a blunt profile to the flowing blood.

The wrinkle-free feature of articles of the present invention canbenefit the performance of tapered stent-grafts. Tapered grafts arewidely used in the treatment of aortoiliac disease. The presentinvention, which can include or not include a tapered stent and/orcover, can be implanted inside a tapered vessel without exhibitingwrinkles in the cover. That is, regardless of the shape of the startingmaterials, the device of the present invention can conform to become atapered self-expanding stent-graft when deployed within a tapered bodyconduit. This allows tapered body conduits to be treated withnon-tapered devices that are easier and less expensive to construct,without deploying an improperly sized stent-grafts. This also allows fora wider range of effective deployable sizes and shapes without the needto increase the number of different configurations of products.

The present invention has particular value in very demanding, smallcaliber stenting applications. These are applications in which a coveris needed to either protect against plaque or other debris from enteringthe blood stream after balloon angioplasty or to seal an aneurysm.Perhaps the most demanding applications are those involving thetreatment of carotid and neural vessels where even small wrinkles in thestent cover may create a nidus for thrombosis. Given the sensitivity ofthe brain, the consequences of such thrombus accumulation and possibleembolization can be dire. Not only does the present invention overcomethe challenging problem of providing a wrinkle-free cover in a viablestent-graft, it accomplishes this with a surprisingly minimal amount ofcovering material. It was unanticipated that such a distensible, thin,and low mass material could satisfactorily perform as a stent covering.

The following examples are intended to illustrate how the presentinvention may be made and used, but not to limit it to such examples.The full scope of the present invention is defined in the appendedclaims.

EXAMPLES

To evaluate the examples, the following test methods were employed.

Test Methods

Assessment of Wrinkles

Stent-graft device covers were visually examined without the aid ofmagnification at ambient temperatures. Microscopic examination might bewarranted for very small devices. The ends of devices were securedwithin a hollow DELRIN® acetal resin block in order fix the longitudinalaxis of the device at an angle of about 45° above horizontal whichenabled viewing the inner surface of the stent-grafts. The devices werepositioned to allow examination of free edge of the device and stentopenings nearest the ends of the device. Stent-grafts that were notfully deployed were constrained inside rigid tubes during examination.Fully deployed devices were submerged in an about 37° C. water bathprior to examination.

Alternatively, optical or scanning electron microscopy could be used tolook for the presence or absence of wrinkles.

Dimensional Measurements

Stent and covered stent device outer diameters were measured with theaid of a tapered mandrel. The end of a device was slipped over themandrel until the end fit snuggly onto the mandrel. The outer diameterof the device was then measured with a set of calipers. Optionally, aprofile projector could be used to measure the outer diameter of thedevice while so placed on the mandrel.

The fully deployed outer diameter was measured after allowing theself-expanding device to fully deploy in a 37° C. water bath for 30seconds, then measuring the device diameter in the water bath in themanner previously described.

For devices constrained inside constraining means having a roundcross-section, the device outer diameter in the constrained state wastaken to be the inner diameter of the constraining means.

In order to examine a device at some percentage of the fully deployeddiameter of the device, the fully deployed diameter must first be known.A length of a device can be severed from the entire device and its fullydeployed diameter can be measured. For example, a length of the devicecan be released from the delivery catheter and its diameter measuredafter being fully deployed in a 37° C. water bath.

Tensile Break Load, Matrix Tensile Strength (MTS), and Young's ModulusDeterminations

Tensile break load of the film was measured using a tensile test machine(Model 5564, Instron Corporation, Norwood, Mass.) equipped withflat-faced grips and a 10 N and 100 N load cells for the transverse andlongitudinal values, respectively. The gauge length was 1 inch (2.54 cm)and the cross-head speed was 1 in/min (2.54 cm/min). Each sample wasweighed using a Mettler AE2000 scale (Mettler Instrument, Highstown,N.J.), then the thickness of the samples was measured using a snap gauge(Mitutoyo Absulute, Kawasaki, Japan). A total of ten samples weretested. Half were tested in the longitudinal direction, half were testedin the transverse (i.e., orthogonal to the longitudinal) direction andthe average of the break load (i.e., the peak force) was calculated. Thelongitudinal and transverse MTS were calculated using the followingequation:MTS=(break load/cross-section area)*(density of PTFE)/bulk density ofthe film), wherein the density of PTFE is taken to be 2.2 g/cc.

Young's modulus was determined from tensile test data obtained using atensile test machine (Model 5500, Instron Corporation, Norwood, Mass.).The test was performed using a sample gauge length of 1 inch (2.54 cm)and a cross-head speed of 1 in/min (2.54 cm/min). A total of ten sampleswere tested. Half were tested in the longitudinal direction, half weretested in the transverse (i.e., orthogonal to the longitudinal)direction.

Inventive Example 1

Tubular, self-expanding nitinol stents constructed using the pattern asdescribed in FIG. 4 of U.S. Pat. No. 6,709,453 to Pinchasik et al., wereobtained. The stents had an outer diameter of approximately 8 mm andlengths of about 44 mm. Six sections about 15 mm in length were cut fromthe stents. Each of the six sections was processed in the followingmanner. The stent was dip-coated with PMVE-TFE, a liquefiedthermoplastic fluoropolymer as described in Example 5 of US PatentApplication 2004/0024,448 of Chang, et. al.

A short piece of silver-plated copper wire (approximately 0.5 mm indiameter) was fashioned into a hook and used to suspend the stent. Thestent was submerged in a 3% by weight solution of PMVE-TFE and FC-77solvent (3M Fluoroinert, 3M Specialty Chemicals Division, St Paul,Minn.). The dipped stent was removed from the solution and air-dried.The hook attached to the opposite end of the stent and the dippingprocess was repeated. The stent was next dipped in a 2% by weightsolution of the fluoropolymer and the solvent, then air-dried. Onceagain, the hook was attached to the opposite end of the stent and thestent was again dipped into the 2% solution. This dipping process,therefore, consisted of four total dips, which yielded a uniform anduninterrupted layer of thermoplastic fluoropolymer on the stent struts.The amount of material applied weighed approximately 0.01 grams asdetermined by weighing the stent before and after the dipping process.

A stent covering was made as follows. A 4.0 mm stainless steel mandrelwas obtained. A 4 mm inner diameter thin-walled (wall thickness of about0.1 mm) ePTFE tube was fitted over the mandrel. The purpose of this tubewas to later assist in removing the stent cover from the mandrel. Next,a spiral wrapping of ribbon of polyimide sheeting (KAPTON®, Part NumberT-188-1/1, Fralock Corporation, Canoga Park, Calif.) was applied on topof the ePTFE tube to completely cover a 75 mm length of the graft.

A thin ePTFE film with the following properties was obtained: width ofabout 50 mm, matrix tensile strength in the longitudinal direction ofabout 256 MPa, matrix tensile strength in the transverse direction ofabout 31 MPa, a thickness of 0.02 mm, and a density of about 0.39 g/cc.(The tensile strengths in the longitudinal and transverse directionswere 45 MPa and 5 MPa, respectively.) Young's modulus values of the filmin the longitudinal and transverse directions were 282 MPa and 1.9 MPa,respectively. An approximately 80 mm length of the film was applied ontop of the polyimide sheeting in the axial direction of the mandrel suchthat the ends of film were in direct contact with the thin-walled ePTFEtube. The corners of these ends were heat bonded to the thin-wall tubewith the use of a local heat source (Weller Soldering Iron, modelEC200M, Cooper Tools, Apex, N.C.) set to 343° C. With the film tacked inplace in this manner, one layer of the film was wrapped about thecircumference of the mandrel. Wrapping of the film was performed underminimal tension in order to avoid stretching the film. Approximately a 2mm width of overlap region was created. The film layers in this overlapregion were heat bonded together with the soldering iron set to 343° C.to form a seam. For this construction, therefore, the longitudinaldirection of the film, which was its high strength direction, wasoriented along the length of the mandrel. The weaker, transverse, filmdirection was oriented in the circumferential direction of the mandrel.

A second layer of polyimide film was helically wrapped on top of theePTFE film, completely covering it. This entire assembly was then placedin a forced air oven (Model NT-1000, Grieve Corporation, Round Lake,Ill.) set at 370° C. The assembly was removed from the oven after 7minutes and allowed to cool. After cooling, the outer wrap of polyimidefilm was removed. The film tube, inner layer of polyimide film, and thethin-walled ePTFE tube, together, were carefully removed from themandrel. The thin-walled ePTFE tube was everted, thereby removing itfrom the polyimide film. The polyimide film was then carefully removedfrom the ePTFE film tube.

The stent and film tube were next assembled into a stent-graft. TheePTFE film tube was inserted inside a 60 mm long glass tube having aninner diameter of 4 mm and a wall thickness of 1 mm such that both endsof the film tube extended beyond the ends of the glass tube. The ends ofclosed forceps were then used to spread the ends of the film tube byplacing them inside each end of the tube and then opening them. The filmtube ends were everted over the outside of the glass tube. The film wastacky enough to secure the ends to the surface of the tube, therebyholding the wrinkle-free film tube in place. The glass tube with theePTFE film tube inside it was placed in a conventional freezer set atapproximately −15° C. Tools that would later be used to create thestent-graft, namely a set of tweezers and an iris-type stent crimpingdevice, such as taught in US 2002/0138966 A1 to Motsenbocker, were alsochilled in the freezer compartment.

The chilled crimping device was used to reduce the diameter of theadhesive-coated stent uniformly along its length. The outer diameter ofthe stent was reduced to about 3 mm. Using chilled tweezers, thefollowing procedure was performed inside the freezer compartment. Thestent was removed from the crimper and transferred into the ePTFE filmtube that was inside the chilled glass tube. The glass tube, film tubeand stent were then removed from the freezer and allowed to warm toambient temperature. The stent, by virtue of its shape memorycharacteristics, self-expanded as the assembly warmed. In doing so, thestent exerted radial force against the film tube, creating intimatecontact between the stent and the film-tube along the length of thestent.

Next, the stent cover was bonded to the stent. This assembly, stillconstrained by the 4 mm inner diameter of the glass tube, was thendipped in a container of FC-77 solvent for 40 seconds in order toactivate the adhesive. The assembly was then allowed to dry forapproximately 30 minutes while being warmed to 40° C. through the use ofa halogen lamp. The assembly was allowed to cool to ambient temperature.In this way, a stent-graft device was created.

The stent-graft device was pushed to one end of the glass tube until theend of the stent was flush with the end of the glass tube. The ePTFEcovering was trimmed flush with the stent. The process was repeated totrim the opposite end of the stent-graft. With the stent-graft stillinside the glass tube, the device was inspected to ensure thorough anduniform bonding between the stent cover and the stent and to verify theabsence of wrinkles in the covering.

The next step entailed loading the stent-graft into a delivery system.The stent-graft device, still constrained by the glass tube, was chilledin a freezer as previously described. The device was then transferred toinside a chilled iris crimper and further radially crushed to reduce itsouter diameter to the desired delivery profile (i.e., crushed outerdiameter), which was about 2 mm. The device was then transferred fromthe crimper into its intended delivery system. Thus, the device wasprevented from self-expanding to its fully deployed outer diameterduring the assembly and loading processes.

The resultant stent-graft device had a delivery profile of about 2 mmand a fully deployed outer diameter of 8 mm. Photographs were taken ofthe device at various stages of deployment and subsequent re-crushing.The outer diameter of the device was characterized as a percentage ofthe fully deployed outer diameter, which was about 8 mm. The fullydeployed device outer diameter was about 8 mm at both about 37° C. andat ambient temperature. It should be noted that this may not be the casefor other types of nitinol alloys.

FIGS. 3 a through 3 f are photomicrographs showing the inside of the sixcovered stents of this example. One device was transferred from its 2 mmdelivery profile constraining sheath into a hollowed DELRIN® resin blockwith an inner diameter corresponding to about 50% of the fully deployedouter diameter of the device. This 50% of the fully deployed outerdiameter corresponds to the outer diameter at which the device was made.Photomicrographs were taken of the end of the device as previouslydescribed. A representative image is shown as FIG. 3 a. Thisphotomicrograph indicates the absence of wrinkles in the stent covering.Another device was transferred into a hollowed DELRIN® resin block withan inner diameter corresponding to about 60% of the fully deployed outerdiameter of the device. A representative image is shown as FIG. 3 b.This photomicrograph indicates the absence of wrinkles in the stentcovering. A third device was transferred into a hollowed DELRIN® resinblock with an inner diameter corresponding to about 70% of the fullydeployed outer diameter of the device. A representative image is shownas FIG. 3 c. This photomicrograph indicates the absence of wrinkles inthe stent covering. The fourth and fifth stent-grafts were transferredinto hollowed DELRIN® resin blocks with inside diameters of 80% and 90%of the fully deployed outer diameter of the devices, respectively;representative photomicrographs appear in FIGS. 3 d and 3 e,respectively. The coverings were wrinkle-free in both of these states,as indicated in the photomicrographs. The sixth device was fullydeployed in a 37° C. water bath and then examined under a microscope. Arepresentative image is shown as FIG. 3 f. This photomicrographindicates the absence of wrinkles in the stent covering.

Comparative Example 2

Film used in the construction of the six stent-graft devices of Example1 was used to make a stent-graft in accordance with the teachings of theprior art. The cover was applied to a length of a stent of the typepreviously-described. In this case, the cover was attached to the stentin the fully deployed state under ambient conditions. The cover wasapplied in the same manner as described previously. The stent-graftdevice was then transferred to inside a chilled iris crimper aspreviously described and further radially crushed to reduce its outerdiameter to the desired delivery profile (i.e., crushed outer diameter),which was about 2 mm. The device was then transferred from the crimperinto its intended delivery system. Thus, the device was prevented fromself-expanding to its fully deployed outer diameter during the assemblyand loading processes. The resultant stent-graft device had a deliveryprofile of about 2 mm and a fully deployed outer diameter of 8 mm. Thisdevice was deployed within a hollow DELRIN® resin cavity, as describedin Example 1. The diameter of the hole in the block corresponded toabout 50% of the fully deployed diameter of the device. A representativephotomicrograph of the crushed device appears as FIG. 3 g.

The advantage of making the stent-graft device of the present inventionin the above-described manner is clear when comparing FIG. 3 a with FIG.3 g. Both photomicrographs were taken at 50% of the fully deployed outerdiameter. FIG. 3 a, unlike FIG. 3 g, exhibits no wrinkles. FIG. 3 ademonstrates the wrinkle-free benefit of the present invention. On theother hand, FIG. 3 g demonstrates the wrinkles that result from crushinga film tube that was made at 100% of the deployed diameter, then crushedto 50% of the deployed diameter. Note the wrinkles in the leading edgeof the cover in FIG. 3 g.

While particular embodiments of the present invention have beenillustrated and described herein, the present invention should not belimited to such illustrations and descriptions. It should be apparentthat changes and modifications may be incorporated and embodied as partof the present invention within the scope of the following claims.

We claim:
 1. A method of producing a stent-graft, comprising providing astent; applying adhesive to the stent; chilling the stent; reducing thestent in diameter while chilled; providing a tubular cover; insertingthe tubular cover into a tubular constraining device; chilling the coverand the tubular constraining device; inserting the stent that is chilledinside the cover that is chilled; and adhering the stent and covertogether to create the stent-graft.
 2. The method of claim 1, whereinproviding a stent comprises providing a stent having shape memorycharacteristics.
 3. The method of claim 2, wherein providing a stenthaving shape memory characteristics comprises providing a stentcomprising nitinol.
 4. The method of claim 2, further comprisingadhering the stent and cover together at a temperature warmer than thetemperature at which the stent was chilled.
 5. The method of claim 1further comprising compressing the stent-graft to a smaller diameter andplacing into a delivery system.
 6. The method of claim 4, whereinproviding a tubular cover comprises: providing a tubular covercomprising an ePTFE film that possesses a density in a range of about0.2 to about 0.6 g/cc, a matrix tensile strength in a range of about 70to about 550 MPa in a longitudinal direction and in a range of about 15to about 50 MPa in a transverse direction, and a Young's modulus in arange of about 100 to about 500 MPa in the longitudinal direction and ina range of about 0.5 to about 20 MPa in the transverse direction,wherein the longitudinal direction of the ePTFE film is substantiallyaligned with a longitudinal axis of the stent.
 7. The method of claim 4,wherein providing a tubular cover comprises: providing a tubular covercomprising ePTFE film that possesses a density in a range of about 0.3to about 0.5 g/cc.
 8. The method of claim 4, wherein providing a tubularcover comprises: providing a tubular cover comprising ePTFE film thatpossesses a matrix tensile strength in a range of about 150 to about 400MPa in a longitudinal direction and in a range of about 20 to about 40MPa in a transverse direction.
 9. The method of claim 4, whereinproviding a tubular cover comprises: providing a tubular covercomprising ePTFE film that possesses a Young's modulus in a range ofabout 200 to about 400 MPa in a longitudinal direction and in a range ofabout 1 to about 10 MPa in a transverse direction.
 10. The method ofclaim 4, wherein providing a tubular cover comprises: providing atubular cover comprising ePTFE film that possesses a Young's modulus ofabout 300 MPa in a longitudinal direction and about 2 MPa in atransverse direction.
 11. The method of claim 4, wherein providing atubular cover comprises: providing a tubular cover comprising ePTFE filmthat possesses a thickness of about 0.02 mm, a density of about 0.4g/cc, a longitudinal matrix strength of about 260 MPa, and a transversematrix tensile strength of about 30 MPa.
 12. A method of producing astent-graft, comprising providing a stent having shape memorycharacteristics having a stent diameter at ambient temperature; applyingadhesive to the stent; chilling the stent; reducing the stent indiameter while chilled to a diameter smaller than the diameter atambient temperature; providing a tubular cover comprising ePTFE filmhaving a cover diameter smaller than the stent diameter at ambienttemperature; inserting the tubular cover into a tubular constrainingdevice having an inner diameter smaller than the stent diameter atambient temperature; chilling the cover and the tubular constrainingdevice; disposing the stent that is chilled within the tubular coverthat is chilled that is inside the tubular constraining device that ischilled; warming the tubular constraining device, the tubular cover andthe stent to ambient temperature, wherein by virtue of its shape memorycharacteristics, the stent self-expands exerting radial force againstthe tubular cover creating intimate contact between the stent and thetubular cover along a length of the stent; bonding the tubular cover tothe stent forming a stent-graft; chilling the stent-graft whileremaining constrained by the tubular constraining device; removing thestent-graft that is chilled from the tubular constraining device;compressing the stent-graft that is chilled to a smaller diameter; anddisposing the stent-graft that is chilled onto a delivery system. 13.The method of claim 12, wherein providing a tubular cover comprisingePTFE film comprises: providing a tubular cover comprising an ePTFE filmthat possesses a density in a range of about 0.2 to about 0.6 g/cc, amatrix tensile strength in a range of about 70 to about 550 MPa in alongitudinal direction and in a range of about 15 to about 50 MPa in atransverse direction, and a Young's modulus in a range of about 100 toabout 500 MPa in the longitudinal direction and in a range of about 0.5to about 20 MPa in the transverse direction, wherein the longitudinaldirection of the ePTFE film is substantially aligned with a longitudinalaxis of the stent.
 14. The method of claim 12, wherein providing atubular cover comprising ePTFE film comprises: providing a tubular covercomprising ePTFE film that possesses a density in a range of about 0.3to about 0.5 g/cc.
 15. The method of claim 12, wherein providing atubular cover comprising ePTFE film comprises: providing a tubular covercomprising ePTFE film that possesses a matrix tensile strength in arange of about 150 to about 400 MPa in a longitudinal direction and in arange of about 20 to about 40 MPa in a transverse direction.
 16. Themethod of claim 12, wherein providing a tubular cover comprising ePTFEfilm comprises: providing a tubular cover comprising ePTFE film thatpossesses a Young's modulus in a range of about 200 to about 400 MPa ina longitudinal direction and in a range of about 1 to about 10 MPa in atransverse direction.
 17. The method of claim 12, wherein providing atubular cover comprising ePTFE film comprises: providing a tubular covercomprising ePTFE film that possesses a Young's modulus of about 300 MPain a longitudinal direction and about 2 MPa in a transverse direction.18. The method of claim 12, wherein providing a tubular cover comprisingePTFE film comprises: providing a tubular cover comprising ePTFE filmthat possesses a thickness of about 0.02 mm, a density of about 0.4g/cc, a longitudinal matrix strength of about 260 MPa, and a transversematrix tensile strength of about 30 MPa.
 19. The method of claim 12,wherein providing a stent having shape memory characteristics comprisesproviding a stent comprising nitinol.